The present invention is of particular value in a lighting unit designed for functional similarity to an incandescent lamp and wherein the principal source of light is a miniature high pressure metal vapor arc lamp supplemented by a standby filament. The unit includes a compact high frequency power supply for achieving the needed energization and regulation from a conventional 100 volt, 60 Hertz electric supply. An example of such a power supply is described in U.S. Pat. No. 4,151,445--Davenport and Diamond, Instant Light Lamp Control Circuit, Apr. 24, 1979. The arb tube may be a miniature metal halide discharge lamp having a volume less than 1 cc and an input rating from 100 watts to as low as 10 watts. By following the design principles taught in pending application of Cap and Lake, Ser. No. 912,628, filed June 5, 1978, titled "High Pressure Metal Vapor Discharge Lamps of Improved Efficacy", which is assigned like this application, arc tube efficacies are achieved equal to those formerly obtainable only in lamps of ratings of 175 watts or more.
The miniature metal halide arc tube forming part of the lighting unit contains sodium iodide as one of its fill ingredients, as do substantially all commercially available metal diode arc lamps. The loss of sodium atoms by the movement of Na.sup.+ ions through the hot silica of the walls in sodium-containing lamps is well-known. The loss of sodium atoms from NaI frees iodine which can then combine with the mercury in the arc tube to form HgI.sub.2 and this leads to many difficulties such as hard starting and change in color of the emitted radiation. Reference may be made to Electric Discharge Lamps by Waymouth, M.I.T. Press 1971, Chapter 10 for a detailed description of the sodium loss process in metal iodide arc lamps. The solution to the problem which has been adopted by the major lamp manufacturers in the U.S. has been the so-called "frameless" harness as taught in Pat. No. 3,424,935--Gungle et al, 1969, Harness Construction for Metal Arc-Type Lamp. There is good evidence that most of the sodium loss is due to a negative charge on the arc tube walls caused by photoelectric emission from the frame side rods used to support the arc tube within the outer bulb in prior art construction. In the "frameless" construction, there are no side rods running alongside the arc tube and the current return wire for the outer end electrode is a fine piece of tungsten wire, sometimes known as the flying lead, spaced as far away from the arc tube as possible and hugging the curve or bulge in the outer bulb.